Add response.next_request

[lovelydinosaur]

Currently when using allow_redirects=False, we have interfaces for response.next() and response.anext(),
which can be used to issue the next redirect request.

The intent of these is to mirror the interface from requests, which also has a response.next.

However It turns out that I've made an error in what I thought the interface we were mirroring does. When using requests with allow_redirects=False, the response.nextisn't a callable. It's an optional request instance, that can be used with client.send(...) to send the next response.

Since one of our desired aims is for full parity with requests, we ought to be covering that API, and not the callable .next interface that we're currently exposing.

client=httpx.Client()
request=client.build_request("GET", ...)
whilerequestisnotNone:
response=client.send(request, allow_redirects=False)
request=response.next_request

This pull request adds response.next_request as a more clearly named version of the requests API.

In the next version bump I think we should follow up on this PR by removing response.next() and response.anext() as unnecessary bits of API.

[jcugat]

I love how this feature it's just a couple lines of code, brilliant!